What is this strange-looking object? Can you guess what it is? It’s a model of a certain type of matter. Some types of matter are elements, or pure substances that cannot be broken down into simpler substances. Many other types of matter are compounds. The model above represents a compound. The compound it represents is carbon dioxide, a gas you exhale each time you breathe.

What Is a Compound?

A
compound
is a unique substance that forms when two or more elements combine chemically. For example, the compound carbon dioxide forms when one atom of carbon (grey in the model above) combines with two atoms of oxygen (red in the model). Another example of a compound is water. It forms when two hydrogen atoms combine with one oxygen atom. You can learn more about compounds and how they form by watching the video at this URL:

A:
It could be represented by a model like the one for carbon dioxide above. You can see a sample
Figure
below
.

A model of water.

Two things are true of all compounds:

A compound always has the same elements in the same proportions. For example, carbon dioxide always has two atoms of oxygen for each atom of carbon, and water always has two atoms of hydrogen for each atom of oxygen.

A compound always has the same composition throughout. For example, all the carbon dioxide in the atmosphere and all the water in the ocean have these same proportions of elements.

Q:
How do you think the properties of compounds compare with the properties of the elements that form them?

A:
You might expect the properties of a compound to be similar to the properties of the elements that make up the compound. But you would be wrong.

Properties of Compounds

The properties of compounds are different from the properties of the elements that form them—sometimes very different. That’s because elements in a compound combine and become an entirely different substance with its own unique properties. Do you put salt on your food? Table salt is the compound sodium chloride. It contains sodium and chlorine. As shown in the
Figure
below
, sodium is a solid that reacts explosively with water, and chlorine is a poisonous gas. But together in table salt, sodium and chlorine form a harmless unreactive compound that you can safely eat.

Sodium and chlorine combine to form sodium chloride, or table salt.

Q:
The compound sodium chloride is very different from the elements sodium and chlorine that combine to form it. What are some properties of sodium chloride?

A:
Sodium chloride is an odorless white solid that is harmless unless consumed in large quantities. In fact, it is a necessary component of the human diet.

Structure of Compounds

Compounds like sodium chloride form structures called crystals. A
crystal
is a rigid framework of many ions locked together in a repeating pattern. Ions are electrically charged forms of atoms. You can see a crystal of sodium chloride in the
Figure
below
. It is made up of many sodium and chloride ions.

A sodium chloride crystal consists of many sodium ions (blue) and chloride ions (green) arranged in a rigid framework.

Compounds such as carbon dioxide and water form molecules instead of crystals. A
molecule
is the smallest particle of a compound that still has the compound’s properties. It consists of two or more atoms bonded together. You saw models of carbon dioxide and water molecules above. You can learn more about molecules at this URL:
http://www.nyscience.org/marvelousmolecules/marveloussub.html
.

Summary

A compound is a unique substance that forms when two or more elements combine chemically. A compound always has the same elements in the same proportions.

The properties of compounds may be very different from the properties of the elements that form them.

Some compounds form rigid frameworks called crystals. Other compounds form individual molecules. A molecule is the smallest particle of a compound that still has the compound’s properties.

Vocabulary

compound
: Unique substance that forms when two or more elements combine chemically.

crystal
: Rigid, lattice-like framework of many ions bonded together that is formed by some compounds such as table salt (NaCl).

molecule
: Smallest particle of a compound that still has the compound’s properties.

Practice

Investigate compounds at the following URL. Be sure to view the rotating 3-D models of molecules and crystals. Then answer the questions below.